A technology of writing additional information in an optically-readable format such as characters or symbols, bar codes, and two-dimensional codes in an input document or the like using a visible or invisible ink has been widely used in, for example, the field of a ledger sheet process. In the field of the ledger sheet process, a process number and the like are written as the additional information in the ledger sheet with entries already written therein, and the process number and the like written in the ledger sheet are optically read to be utilized for the subsequent processes.
For the correctness in reading, the additional information makes it a rule to be written at a position where no overlapping with other pre-existing entries occurs. However, when the format of a document or the like is unknown, the additional information may be written on the pre-existing entries so as to be overlapped with each other, and thus the additional information may not be correctly read. In addition, when a wide variety of document formats are handled, a region enough to write the additional information may not be acquired in the same position with respect to the whole formats in common. For this reason, there has been a demand for a technology through which a write region of the additional information can be appropriately acquired with respect to the documents of an unknown format and a wide variety of formats, and the additional information written in the region can be correctly recognized.